Nowadays, photo-lithography is generally adopted in processes for production of semiconductor circuits. In a photo-lithographic process, a resist film is imagewise exposed to light so as to generate acid by a photochemical reaction. The generated acid acts on the resist, for example, it induces an elimination reaction of protecting groups of the resist, so that the solubility of the resist is changed to form a pattern.
Actually, however, depending upon the environmental conditions under which the resist is placed, the concentration of acid in the exposed area patterned by the exposure may so vary that the elimination reaction of protecting groups insufficiently proceeds in a practical photo-lithographic process. As a result, the formed pattern often suffers from surface defects such as resist residues, blob defects and blocking defects of holes. Those defects can be sometimes reduced if the resist surface is coated with a water-soluble acidic surface antireflection film.
Meanwhile, in accordance with recent demand for miniaturizing semiconductor circuits, the sizes of resist patterns have been also required to be miniaturized. In order to meet the requirement, it has been studied for lithographic processes to employ light of shorter wavelength, such as, KrF or ArF excimer laser beams. Further, “liquid immersion lithography” is becoming practically used to advance the miniaturization. The liquid immersion lithography is a technique in which liquid medium is inserted between the wafer and a projection lens in the exposure step so that refraction in the liquid medium is utilized to form a high-resolution pattern. As the liquid medium, pure water is often used in recent liquid immersion lithographic processes that are becoming adopted in practical production.
Since the resist is brought into direct contact with the liquid medium such as pure water in a liquid immersion lithographic process, components contained in the resist are liable to dissolve out into the liquid medium. Accordingly, the concentration of acid formed in the exposed resist is more likely to decrease than in a normal lithographic process. However, the aforementioned water-soluble acidic surface antireflection film, which can be provided on the resist surface in a normal lithographic process, cannot be used in the liquid immersion lithographic process because the resist surface is brought into contact with water. In that case, therefore, some measures are taken to prevent the components in the resist from dissolving out. For example, the resist surface is covered with a film insoluble in the liquid medium and/or the resist is made to contain additives that prevent the components from dissolving out.
In a liquid immersion lithographic process employing pure water as the liquid medium, the contact angle of pure water to the resist surface is generally designed to be so large that water drops may not remain on the resist surface. That is for the purpose of speeding up the exposure step. On the other hand, however, if the contact angle is enlarged, it is difficult in the development step to soak the resist with a developing solution containing water as the solvent. As a result, defects are liable to form in the resist. Thus, there is a dilemma in that, although the contact angle is preferably enlarged from the viewpoint of speeding up the procedure, the enlarged contact angle may promote formation of blob defects and blocking defects in hole patterns.
In order to solve the above problems, some processes have been proposed. In each process, the resist is rinsed with a rinse solution after subjected to exposure, heat baking and development with a developing solution. For example, the resist may be rinsed with a rinse solution of low surface tension (Patent document 1). The rinse solution may be warm water or an organic solvent compatible with a surfactant or pure water (Patent document 2), and the surfactant may be a fluorine-containing surfactant (Patent document 3). The rinse solution may contain solvents such as alcohols (Patent documents 4 and 5). Further, the resist may be rinsed with a rinse solution of low viscosity such as hot water (Patent document 6). It is also proposed to add a surfactant having a particular structure into the rinse solution (Patent documents 7 and 8). Those processes are so designed that defects are allowed to form but are removed after development of the resist. After the resist is subjected to exposure and heat baking, a surface modifier may be used to reduce the contact angle of the developing solution to the resist surface (Patent document 9). However, those proposed processes are still so designed as to remove the defects in the developed resist afterward but as not to solve the essential problem of acid concentration decrease in the exposed area patterned by the exposure.